Electric-power transmission is the bulk transfer of electrical energy from generating power plants to electrical substations. Most transmission lines use high-voltage three-phase alternating current (AC) that requires three conductors (three cables). Three-phase alternating current (AC) is used because it can be transmitted at high voltages (110 kV or above) or extra high voltage (230 kV to 765 kV) to reduce the energy lost in long-distance transmission. Using transformers, the electricity can then be efficiently reduced to sub-transmission level (33 kV to 132 kV) and distribution level (3.3 kV to 25 kV). Finally, the energy is transformed to low voltage (240V or 440V) for use in homes and small business. Due to the evolution of power systems, the transmission voltages, sub-transmission voltages, and distribution voltage ranges overlap somewhat.
Transmission lines may be supported by poles or structures and run overhead or to a lesser degree may be run underground. Some power lines are directly buried in the ground but, as a standard practice, mission critical underground transmission lines are placed in duct banks. The duct banks are normally at least three feet below ground level and consist of multiple conduits that are located a controlled distance apart in an organized matrix that is encased in concrete. In this disclosure, the words duct, conduit and pipe are interchangeable and have the same meaning. In addition, the word concrete is used interchangeably with thermal concrete, concrete slurry or flowable fill. These materials are used to separate the conduits and conduct heat away from the conduits.
The power cables are placed in concrete-encased duct banks for several reasons. The concrete protects them, primarily from digging, whether with hand tools or with mechanized equipment, such as backhoes. Backfill, roads, railroads and the like are placed on top of the duct bank, resulting in heavy loads on the duct bank. A duct bank backfill material other than concrete may settle unevenly. As the concrete settles, the conduits within the concrete matrix settle in unison. The base spacers in the duct bank are supported by undisturbed earth and resist movement of the settling conduits. Irregular movement caused by settling would otherwise deform or crush the conduits. The encased conduits, arranged in their current volume of usage, are made from PVC, HDPE or FRE (fiberglass-reinforced epoxy). Other conduit materials are used to a much lesser degree. The concrete also acts to dissipate the heat generated by transmission of electric power through the cables.
A typical method that is used to construct an underground power transmission line is as follows:                1. Open cut a trench to the required width, depth and length.        2. If required, reinforce or shore up the walls of the trench to insure that it doesn't cave in during the duct bank installation process.        3. Build the duct bank.                    a. Place base spacers on the bottom of the trench.            b. Top load a row of conduits into the base spacers.            c. Solvent cement the row of conduits to the previously laid section of duct bank.            d. Place an intermediate row of spacers atop the previously laid row of duct spacers or conduits.            e. Top load the next row of conduits into the intermediate spacers.            f. If “e” was the top row of conduits go to “g,” otherwise go to “d.”            g. If desired, place a top row of spacers atop the previously laid row of duct spacers or conduits. This top row of spacers can be used to gage the depth of the concrete cover and aid in the hold down.                        4. Place a hold down mechanism atop the duct bank. A hold down mechanism is required to keep the duct bank from floating when the concrete is poured. An example is to use rebar (reinforcing rods of steel) to tie the duct bank structure to the floor or side walls of the trench.        5. Pour concrete over the duct bank, completely encasing the duct bank. Normally there should be 3 inches of concrete between the bottom of the lower-most conduits and the bottom of the trench, 3 inches of concrete on each side of the duct bank and 3 inches of concrete cover atop the upper-most row of conduits.        6. Allow the concrete to harden.        7. Remove the trench side wall shoring and hold down mechanism as applicable.        8. Backfill the exposed trench opening with the appropriate backfill material. Compact the backfill in lifts as required.        9. Mate the ends of the duct bank with manholes or vaults that have normally already been put in place.        10. Pull the cables down through the manholes or vaults and through the conduits.        11. Restore the surface above the backfill and around the manholes or vaults as required.        
Most overhead and underground transmission lines consist of two sets of three cables (six cables). The double set of cables allows for the rerouting of power through the backup cable set in the event of an emergency situation. For underground duct banks the cable sets may be situated one atop the other or side by side depending on the width of the real estate available and the obstructions encountered along the length of the duct bank.
An electro-magnetic field (EMF) emanates from electric current being transmitted by power cables. Extensive studies have been made on how arrangement of the cables affects EMF from the cables. Electric Power High-Voltage Transmission Lines: Design Options, Cost and Electric and Magnetic Field Levels, J. B. Stoffel, E. D. Pentecost, R. D. Roman and P. A. Traczyk, Environmental Assessment Division, Argonne National Laboratory, ANL/EAD/TM-31, November 1994 (hereinafter “Stoffel”). The EMF is strongest close to the cables and diminishes as the distance from the cable increases. The highest EMF levels for an underground transmission line are directly above the transmission line during maximum current flow. The higher the current flow, of course, the higher the EMF. The study found that placing power lines in a triangular or delta configuration and placing cables closer together led to an apparent cancellation effect and a lower EMF. The study considered a 345 kV line with phases spaced 8 inches (approx. 20 cm) apart and buried in a steel pipe 5 ft (approx. 1.5 m) below the surface. The study reported that electric fields were eliminated in underground cables and that magnetic fields very much reduced at all points except directly above the cable. Previous work found a 94% reduction in magnetic field strength if the conduits were encased in a steel pipe. Stoffel, citing Cost Effectiveness Analysis: Mitigation of Electromagnetic Fields,” from Commonwealth Associates, Inc., 1992.
Some studies have found statistical correlations between various diseases and living or working near power lines. In a residential setting, there is limited evidence of carcinogenicity in humans. Some statistical studies have reported that incidents of childhood leukemia and miscarriages increase when the average exposure to a residential power-frequency magnetic field is above 3 mG (milliGauss) to 4 mG. See, e.g., A Pooled Analysis of Magnetic Fields and Childhood Leukaemia, A. Ahlbom et al., Br. J. Cancer 200; 83:692-8, cited in Childhood Cancer in Relation to Distance from High Voltage Power Lines in England and Wales, G. Draper et al., Br. Med. J., 2005, vol. 330, pp 1290-94. None of the studies or evidence available to date has conclusively proven that exposure to an EMF above 3 to 4 mG is detrimental to human health. Nevertheless, many power utilities and jurisdictions are acting on the side of caution and establishing guidelines and standards that require a “low cost-no cost” mitigation of the EMF emanating from new electric power transmission and distribution lines and installations.
The EMF emanating from power cables may be reduced considerably by phase cancellation using a triangular configuration and reducing the distance between the cables. The phase cancellation technique may require using larger diameter cables to reduce heat generation. Additional EMF reduction may be gained from cross-phase placing of the cables of the six-cable configuration. Stoffel, pp. 16-19, 21-23 and 30.
What is needed is better conduit spacing and a better conduit spacer to minimize electromagnetic emissions from underground cables. The present disclosure includes discussions of systems and methods to minimize these emissions in an economical manner.